1903.] Matthew, Fauna of Titanotherium Beds of Montana. 211 



postero-external cusp, and external basal cingulum rising to 

 a small heel behind. The second molar is a minute crownless 

 stump. 



Measurements. 



P4_m2 10.2 mm. 



P4 longit 4-1 



" transv 1.9 



Mi longit 5.9 



" transv 2.3 



Depth of jaw beneath nij 6. 



A larger Musteline is indicated by part of a jaw, No. 9621, 

 with p^ and the roots of the front teeth in it. It is about the 

 size of Cynodictis pater cuius, but the premolars are higher, 

 shorter; the heel smaller, anterior basal cusp absent; and 

 other details join to show that it is a Musteline. 



RODENTIA. 



ISCHYROMYID^. 



Ischyromys veterior, spec. nov. 



The anterior part of a skull and some forty jaws or parts 

 of jaws, upper and lower, represent this species. It is con- 

 siderably smaller than a series of specimens from South 

 Dakota, which agree well with Leidy's types of 7. iypus, and 

 the teeth are narrower throughout with higher cusps. It is 

 much closer to the variety or separate species from Colorado, 

 I. cristatus (Cope, 1872), which Cope has referred to I. typus 

 and described and figured in 'Tertiary Vertebrata.' From 

 this species I find a constant distinction in the last molar, 

 which in the Montana jaws has always a narrow heel with the 

 last crest imperfect internally, while in all the Colorado speci- 

 mens the heel is as wide as the rest of the tooth, and the third 

 (last) crest perfectly developed. 



In the upper teeth a corresponding difference is to be seen 

 in the last molar, and also the valley between the anterior 

 and posterior inner cusps is well marked on all the teeth, dis- 

 tinct nearly to the base of the enamel, while in the specimens 



